A class of large diameter afferent nerve fibers innervate the otolithic or otoconial organs, the saccule, utricle, and lagena (of non-mammals) and the three semicircular canal cristae in a spatially selective manner. Each of these nerve fibers innervate several sensory hair cells. The physiological response of each fiber presumably results from combining inputs from all innervated hair cells, and physiological dogma presumes that each hair cell that synapses on an afferent fiber has roughly the same potential for influencing action potential generation of that fiber. By using a cytochemical cation-binding procedure that identifies the axonal locations where action potentials may be initiated, preliminary results suggest that an individual hair cell synapsing with these large diameter fibers may directly generate a regenerative potential if the post-synaptic potential produced is sufficiently large. Synapses of adjacent hair cells do not stain, suggesting that other pre-synaptic hair cells may not have the same capacity. Initiation of an action potential at the post-synaptic membrane would insure orthodromic conduction of hair cell responses through the intraepithelial internodal segment of the terminal to the axon proper. This proposal will study the cation binding of post-synaptic membrane in vestibular end-organs of fish, frogs, birds, and mammals. The questions to be considered are (1) the spatial distribution of cation binding post-synaptic membrane; (2) the phylogenic distribution of cation binding post-synaptic membrane in the saccule and utricle of the 4 vertebrate groups named previously; and (3) the relationship between the hair cells synapsing onto a single branch of an eighth nerve fiber and cation- binding post-synaptic membrane.